How common is it for airport officers to steal valuables from luggage?

I've heard that this happens frequently with the TSA in the United States

You may have heard wrong. In 2014 the US airport with most TSA complaints filed was JFK in New York with 891 complaints. In the same year the airport handled 53.3 million customers. It's also fair to assume that not all claims are based on a real event (since it's an easy way to make a quick buck) and the TSA accepts roughly 1/3 of all claims.

I don't think this qualifies as "frequent" as the chances of an incident much less than 1 in 100000. It's often covered in the media in a sensationalist way but only since it makes a good story, not because there is a lot of substance to it. I travel between 100k and 200k miles a year and never had a problem.

Of all the travel threats to your valuable electronics, security screening is a minor one.

It has little to do with the payment level of the country, but with what they get away with.

In the US, the urban knowledge is that laptops, iPads, etc. get nearly always stolen. That might or might not be true, but our company policy is to never put such electronics in the luggage, and never accept a forced checking of your carry-on at the boarding (we are asked to miss the flight if they try to force us). That implies to me that they see a major risk.

There is little you can do as the transport regulations allow them to walk away without legal consequences (you can't sue them or send police). The airlines just reimburse you 20 $ per pound (the maximum), and everyone has learned to live with it. The employees know that of course, and are well aware that it is basically risk-free for them to steal.

I have travelled a lot over the years by a variety of means: trains, planes, busses, boats, etc. Also, many locations: Europe, North American, and Asia. I have suffered a few thefts but none were during flights. The most transport related one was while buying a train ticket in Calcutta (as it was called when I was there).

However, I adopt the policy suggested by Zozor: don't check anything very valuable. So, maybe I have not had stuff stolen from my baggage because I am lucky, because I have only encountered honest airline staff, or because there was never anything worth stealing.

I don't even bother locking my checked in baggage. Since there is nothing very valuable there, I would prefer that someone looked without damaging the case rather than slashed or otherwise forced it open.

Edit (I amended that last line since I just fear that my bag could be slashed by a thief, I don't know actual incident of this happening).